1. Field of the Invention
The present invention relates to a display device. More particularly, the invention relates to a liquid crystal display device in which a countermeasure against afterimage phenomenon is taken, with the afterimage phenomenon occurring when a color filter is formed over a substrate where a pixel electrode is formed.
2. Description of the Related Art
Since liquid crystal display devices can be reduced in thickness and weight, their application use has been extended to various fields including large-scaled devices such as television, and mobile phones and DSC (Digital Still Camera). On the other hand, the liquid crystal display devices have a problem of view angle characteristics. The view angle characteristic is a phenomenon in which brightness or chromaticity changes between a case in which a screen is observed from the front and a case in which the screen is observed in an oblique direction. For the view angle characteristics, an IPS (In Plane Switching) system for operating liquid crystal molecules by electric field in a horizontal direction has excellent characteristics.
A conventional liquid crystal display device includes a TFT substrate having a pixel electrode, thin film transistors (TFT), etc. formed in a matrix; a counter substrate disposed in facing relation to the TFT substrate and having a color filter, etc. formed at portions corresponding to the pixel electrodes of the TFT substrate; and liquid crystals put between the TFT substrate and the counter substrate. Images are formed by controlling the transmittance of light by liquid crystal molecules for every pixel.
However, in the conventional liquid crystal display devices described above, the TFT substrate and the counter substrate have to be positioned accurately. Therefore, the positioning step drives up the manufacturing cost of the liquid crystal display device. Further, since complete positioning is not possible between the TFT substrate and the counter substrate, a positioning margin is taken in view of design. To take the margin, since it is necessary to increase the area for a black matrix, this reduces the transmittance of a liquid crystal display panel to cause loss in the brightness of a screen. Further, formation of a screen having high fineness is also limited.
Then, a technique of forming a color filter on a TFT substrate has been developed. If the color filter is fabricated into the TFT substrate, the pixel electrode and the color filter can be positioned, for example, by a photolithographic step. Therefore, the accuracy is outstandingly higher compared with that in positioning between the TFT substrate and the counter substrate. Further, since the step of forming the color filter requires an identical step both in a case in which the color film is formed on the counter substrate and in a case in which the color film is formed on the TFT substrate, the number of step of forming the color filter is not increased.
Accordingly, by using a system for fabricating a color filter to a TFT substrate (color filter on array: hereinafter referred to as COA), manufacturing cost can be reduced, and the screen brightness can be improved by improving the transmittance of the liquid crystal display device and a screen having high fineness can be achieved.
On the other hand, the liquid crystal display device involves a phenomenon of DC afterimage. This is a phenomenon that electric charges are accumulated on an alignment film when a certain image is displayed for a predetermined time and the image is observed as if it were burnt to the screen for a certain time. The lasting time of the DC afterimage can be shortened by reducing the electric resistance of the alignment film.
Since a backlight is used in the liquid crystal display device, the electric resistance of the alignment film during operation can be reduced by using a photoconductive material for the alignment film. Accordingly, a photoconductive alignment film is used in many recent liquid crystal display devices. Materials showing a photoconductive property as the alignment film include, for example, SE 6414 manufactured by Nissan Chemical Co.
However, in the COA, since the color filter is formed on the side of the backlight with respect to the alignment film, the intensity of a light from backlight reaching the alignment film is less than that in the conventional device. Accordingly, no sufficient photoconductivity can be provided to the alignment film and the resistance of the alignment film during operation cannot be reduced sufficiently. Accordingly, the DC afterimage causes a serious problem with the COA.
The alignment film absorbs more light at shorter wavelength because light at a short wavelength contributes to the photoconductivity. As the transmittance lowers at a short wavelength, there occurs a phenomenon that more light on the side of longer wavelength transmits through the alignment film or transmits through the liquid crystal display panel, and the image is shifted to the side of red color.
JP-A-2011-237571 describes a configuration for increasing the thickness of color filters in a COA in the order of a red filter, a green filter, and a blue filter, thereby preventing shifting of a screen image to the red color. Alternatively, it describes another configuration for increasing the size of pixels in the order of a red filter, a green filter, and a blue filter, thereby preventing the screen image from shifting to the red color.